Award: OCE-2023509

Benthic primary producers on reefs, including corals and algae have been shown to release significant amounts of organic matter into the surrounding water. However, organic matter does not typically accumulate in healthy reef systems due to the tight coupling between production and removal processes. Our research on the reefs of Moorea, French Polynesia, has shown that the microbial community plays a crucial role in degrading locally produced organic matter and may also be important for utilizing compounds that are transported onto the reef from offshore environments. This efficient uptake allows the reef to retain nutrients and provides energy and cellular building blocks to reef organisms. In this work, we used a well-developed field-based experimental ecosystem to produce diverse labile dissolved organic matter (DOM) pools. We then identified compounds and compounds classes that were unique to specific benthic producers and subsequently examined how microbial degradation changed the composition and diversity of newly produced DOM. The impact of this work is that it will be able to define the lability of numerous individual compounds and broader compound classes that participate in the upper ocean carbon cycle and provide new mechanistic insights into DOM-microbe interactions through both producer-specific and consumer-specific lenses. A second goal of our research was to use the molecular level identifications from our experimental system to track specific compounds in situ and to compare our experimental designations of lability with field distributions of the same compounds. We achieved this through field surveys where we examined DOM dynamics across habitat gradients on more realistic timescales. The figures highlight some of the primary experimental and field data collection efforts and outcomes. Using a field-based experimental approach (Figure 1A), unique compounds released by benthic primary producers were detected and their structures were validated using both mass spectrometry and authentic standards (Figure 1B). Subsequent experiments further categorized these compounds into more or less- labile categories. Using initial mass-spectrometric based identification, standard compounds unique to specific benthic producers and exhibiting different lability in previous experiments were purchased and used in controlled degradation experiments with microorganisms specific to the backreef (Figure 2A). By measuring changes in the abundance of specific compounds over time (Figure 2B) and monitoring oxygen drawdown and cell proliferation, we were able to determine whether these compounds were labile over the lifetime of our experiments. Purchased compounds also served as authentic standards and we were able to unequivocally identify their presence in a variety of environmental metabolomic datasets from our controlled, DOM production experiments in Moorea and also in DOM samples isolated from other oceanic regions. Thus, results from the current work are relevant for DOM cycling globally. Finally, by combining these experimental approaches and comparing them with field surveys that followed offshore waters as they entered the reef and moved across the backreef (Figure 3A), we were able to determine how specific primary producers were influencing the DOM accumulating on the reef at a molecular level (Figure 3B). As we think more broadly about the role of DOM in sequestering carbon dioxide, this study, by identifying longer and shorter lived molecules and their broad compound class affiliations and linking them to specific sources and removal pathways, provides structural models that may be used to track the carbon dioxide removal potential of marine ecosystems. In terms of broader impacts, our past and ongoing work centering the role of microbes in coral reef ecosystem science has consistently addressed the importance of microbial processes in ecosystem-based management. Our work particularly has been important for addressing the ongoing problem of phase shifts from coral to algal dominance in tropical reefs. The collaborative project has trained 5 PhD students overall, and this project, specifically, has directly trained three, diverse, graduate students. Results have also been disseminated broadly to the public and at scientific conferences. The analytical approaches pioneered by the graduate students involved in this project have also been shared widely with the relevant research communities. The students have been tireless in educating their peers all over the world on these methods and as such, have played an important role in moving the entire field of environmental metabolomics forward. Last Modified: 04/20/2024 Submitted by: LihiniIAluwihare